A fast, confident, dominant CB who is truly good at passing.
Look at our 2011 final against Barca for instance, Rooney was provided with specific instruction to keep pressing Busquets and it worked great for the first ten minutes, then Pique stepped up and start taking the ball and passing it around between himself, Alves, Puyol and Busquets, just give and take and give and take. Our press ended pretty soon afterwards.
Neither Maguire nor Lindelof can do that because they takes many touches and invites pressure, then either crap their pants and put it to De Gea for a hoof or surrender possession, and when our DM and fullbacks are pressed none of them is confident enough to step out, receive the ball and move up the pitch, drawing the press with them before passing it out of danger, because they have no pace for recovery.
As it is though, our counter gameplan is still good enough, yesterday was just an unfortunate case of us conceding from the press but failing to exploit it since both our creative CMs were off with their final ball, then the freak injuries happened.